To continue the report… the 3g modem was never stable under beta2 as it is/was with 3.105.
Unfortunately, under beta4 of 3.201 the modem is completely unusable.
Starting each time from a clean firmware (no settings retained), I have not yet managed to make the USB modem work for longer than 20 minutes and then only for just one time (did not work after reboot or ever again).
I have spent an hour trying every approach I could think of to make it work again only once but I cannot make it happen.